The present invention relates to a coating method and apparatus in which a desired coating solution is supplied to an extrusion-type coating head and the coating head applies the coating solution to a running support. More particularly, the invention relates to a coating method and apparatus suitable for the manufacture of a magnetic recording medium in which a coating solution such as a magnetic coating solution is applied to the surface of a belt of paper or an elongated web (support) of soft synthetic resin or the like which is being run.
Heretofore, a magnetic recording medium such as a magnetic tape or a photographing film has been formed by applying a coating solution, selected according to the purpose of use, to the surface of a support, drying the support thus treated, and cutting the support to a desired width and length. The term "support" as used herein is intended to mean a belt-shaped material made of a macromolecular compound such as polyethylene terphthalate, cellulose acetate, polyimide or polyamide, paper, copper or metal foil. The "coating solution" includes magnetic material dispersion solutions, photo-sensitive material coating solutions, heat-sensitive material coating solutions, and macromolecular molten solutions.
A coating apparatus using such a coating solution may use an extrusion-type coating head as disclosed, for instance, in Japanese Patent Application (OPI) No. 84771/1982.
The structure of an extrusion-type coating head and a coating method using the coating head will be discussed with references to FIGS. 6 through 8.
A coating solution A is supplied through a coating solution supplying device 3 such as a pipe into a pocket 2 formed in an extruder 1. The pocket 2 is substantially circular in cross section; that is, it is a solution pool whose length is substantially equal to the width of the extruder 1. The effective length of the pocket 2 is, in general, equal to or slightly longer than the coating width.
A slot 44 is formed in the extruder 1 in such a manner that it is communicated with the pocket 3, thus providing a flow path for the coating solution A. The length of the slot 4 is substantially equal to that of the pocket 2.
The pocket 2 is filled with the coating solution A applied through the coating solution supplying device 3 under pressure, as a result of which the coating solution A is caused to flow from the pocket 2 towards the outlet with a uniform liquid pressure distribution.
The extruder 1 has a doctor edge 5 located downstream of a support 7 to which the coating solution A is applied, and a back edge 6 located upstream of the support 7.
The levels of the end faces of the edges 5 and 6 are established depending on the configuration, curvature, etc. of the support 7, for instance, as shown in FIGS. 6 and 7.
The extrusion-type coating heads thus constructed are arranged according to the actual use. For example, as shown in FIG. 6, a coating solution A is applied to a support 7 which is run while being supported by a back-up roller 11. As shown in FIG. 7, a coating solution A is applied to a support 7 which is not backed up. As shown in FIG. 8, a coating solution is applied to a support with the aid of rollers 12 and 13. In each case, the coating solution A is supplied to the pocket 2 through a solution delivering device such as a pump and a coating solution supplying device such as a pipe.
However, if dust or the like is mixed in the coating solution A, it may scratch the support 7 or make the coated surface of the support uneven. In such instances, the resultant product may be unacceptable.
In order to overcome this difficulty, i.e., to remove the dust or reduce the amount of dust, heretofore a filter has been provided in the path of the coating solution supplying device.
However, relatively large particles can still pass through the conventional filter, or large particles formed in the coating solution supplying line between the filter and the pocket 2, such as deposits stuck to the inner walls of the pipes, can be delivered into the pocket 2 together with the coating solution A.
These large particles can be trapped between the support 7 and the end of the extruder 1, thus forming longitudinal stripes on the coated surface of the support 7.
In the case where a thin film layer is formed on the support by applying a coating solution A thereto, the gap between the support and the end face of the extruder is so small that the probability of trapping large particles therebetween, which results in the formation of longitudinal stripes on the coated surface of the support, is increased.